Method of coating a luminescent material

ABSTRACT

A luminescent material coated with a crystalline layer of a metal oxide M 2  O 3  wherein a metal M is chosen from the group formed by Y and Al is shown. According to this method the luminescent material is first introduced into a solution of a salt of the metal M, then a compound of the metal M is deposited on the luminescent material through homogeneous precipitation, and subsequently the luminescent material is separated from the solution, dried, and heated. It is achieved thereby that the luminescent material will absorb mercury to a low degree only, and that accordingly the color point of a low-pressure mercury discharge lamp whose luminescent layer comprises a luminescent material coated in accordance with the invention only shows a small shift as the lamp ages.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 08/512,434, field Aug. 8,1995.

BACKGROUND OF THE INVENTION

The invention relates to a method of coating a luminescent material witha layer of a metal oxide M₂ O₃ in which a metal M is chosen from thegroup formed by Y and Al. The invention also relates to a coatedluminescent material and to a low-pressure mercury discharge lamp.

A method as mentioned in the opening paragraph is known from EuropeanPatent EP 0 488 330. Small particles of the metal oxide and theluminescent material are brought together in an aqueous environment inthe known method. After homogenization through stirring, the solidsubstance is separated from the liquid and dried. In the known method,the metal oxide/luminescent material ratio is so chosen that the metaloxide covers only a comparatively small portion of the surface area ofthe luminescent material. The use of the luminescent material obtainedby the known method in a low-pressure mercury discharge lamp led to animprovement of the lumen maintenance of the low-pressure mercurydischarge lamp compared with that of lamps in which uncoated luminescentmaterials are used. This improved maintenance is probably due to a lessstrong adhesion of HgO to the surface of a luminescent material coatedwith metal oxide. Besides adsorption of HgO at the surface of theluminescent material, adsorption and absorption of Hg also occurs in alow-pressure mercury discharge lamp. This absorption of Hg has a numberof major disadvantages. First, Hg disappears from the plasma as thelow-pressure mercury discharge lamp ages. This decrease in the quantityof Hg in the lamp plasma renders it necessary to introduce acomparatively large quantity of Hg into the lamp in the manufacture ofthe low-pressure mercury discharge lamp. The luminous efficacy of thelow-pressure discharge lamp, however, is comparatively low with thiscomparatively great quantity of mercury. Other disadvantages of themercury absorption are the fact that the optical properties of theluminescent material are changed thereby, while the presence of Hg alsoadversely affects the recycling possibilities of the luminescentmaterial.

SUMMARY OF THE INVENTION

The invention has for its object to provide a method of coating aluminescent material with a metal oxide which leads to a product whichhas a comparatively low mercury absorption when used in a low-pressuredischarge lamp.

According to the invention, a method as described in the openingparagraph is for this purpose characterized in that, in that order, theluminescent material is introduced into a solution of a salt of themetal M, a compound of the metal M is deposited on the luminescentmaterial through homogeneous precipitation, the luminescent material isseparated from the solution, is dried, and is heated.

It was found to be possible to provide luminescent materials with aclosed or substantially closed crystalline, light-transmitting layer bya method according to the invention. The absorption of Hg by theluminescent material is considerably reduced thereby. It was also foundthat the metal oxide layers provided by a method according to theinvention adhere well to the luminescent material, so that theluminescent material provided with a metal oxide layer allows itself tobe applied to the wall of a lamp vessel satisfactorily by a known methodemploying a slurry.

In the case of a Y₂ O₃ layer it was found that part of the Y atoms maybe replaced (up to approximately 5%) by one or several rare earthmetals. Thus it was found, for example, that a Y₂ O₃ layer of which 5%of the Y atoms was replaced by europium is capable of achieving asimilar reduction in the Hg absorption. The partial replacement of Y byone or several rare earth metals may be realised in that the Y salt usedin the method according to the invention is mixed with a quantity ofsalts (soluble in the same solvent) of the relevant rare earth metals.

It is noted that European Patent EP 0 160 856 describes a method inwhich an Al₂ O₃ layer is provided on a luminescent material by chemicalvapour deposition. Chemical vapour deposition is a comparativelyexpensive process. In addition, layers provided by this method areindeed closed, but they are amorphous, so that the adhesion of the Al₂O₃ layer to the luminescent material is comparatively weak. As a resultof this, for example, the layer may detach itself partly from theluminescent material when a slurry is formed with the product in thismethod in order to apply the product to the wall of a lamp vessel inthis manner. It was found for the luminescent material Zn₂ SiO₄ :Mncoated with an Al₂ O₃ layer that the emission properties of theluminescent layer changed when the layer was rendered crystallinethrough a heat treatment between 700° C. and 850° C. This change in theemission properties of the luminescent material was probably due todiffusion of Zn and Mn ions from the luminescent material into the Al₂O₃ layer, and of Al ions from the Al₂ O₃ into the luminescent material.The change in the emission properties which occurs reduces theapplication possibilities in low-pressure mercury discharge lamps of theproduct obtained by the method described in EP 0 160 856.

It was found to be advantageous in the use of a method according to theinvention to heat the luminescent material after drying first in air ata temperature of between 200° C. and 600° C., and then in a reducingatmosphere at a temperature of between 600° C. and 1200° C. Any organicligands of the metal M are oxidized during heating in air. Heating at ahigher temperature in a reducing atmosphere achieves the conversion ofthe layer from an amorphous to a crystalline, oxidic phase. The reducingatmosphere prevents any activators present in the luminescent materialfrom being oxidized, which would lead to a change in the emissionspectrum of the luminescent material. It was found that a maximumdensity of the layer can be achieved, when M=Y, in that heating in areducing atmosphere is carried out at a temperature of between 850° C.and 950° C. When M=Al, a maximum layer density was found upon heating ina reducing atmosphere at a temperature of between 950° C. and 1100° C.These heat treatments did not cause any measurable diffusion of ionsfrom the layer into the luminescent material, or of ions from theluminescent material into the layer, so that the emission properties ofthe luminescent materials were substantially unaffected.

The thickness of the layer formed by the metal oxide M₂ O₃ is preferablychosen to lie within the range of 25 nm-100 nm. With comparatively thinlayers, the Hg absorption further decreases as the layer becomesthicker. This is counterbalanced by the disadvantages of a thickerlayer: higher cost and a longer time required for its manufacture.

The method according to the invention is highly suitable for coatingbarium-magnesium aluminate activated by bivalent europium, called BAMhereinafter. Among the luminescent materials used in the luminescentlayers of low-pressure mercury discharge lamps with emission maxima inthree spectral regions, BAM shows a comparatively high mercuryabsorption. As a result, the speed with which the quality of aluminescent layer comprising BAM in low-pressure mercury discharge lampswith emission maxima in three spectral regions deteriorates is largelydetermined by the speed with which BAM absorbs mercury. It was foundthat the quality deterioration of the luminescent layer of such a lampmay be considerably reduced in that from among the luminescent materialforming part of the luminescent layer exclusively BAM is coated with ametal oxide layer.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing, FIGS. 1 and 2 are graphs showing the luminous efficacyand the mercury consumption of BAM coated according to the invention andof uncoated BAM as a function of the number of hours of operation oflow-pressure mercury discharge lamps in which these phosphors wereprovided;

FIGS. 3 and 4 are graphs showing the shift in the x-coordinate andy-coordinate, respectively, of the colour point of the light of theselamps as a function of the number of hours of operation,

FIGS. 5, 6 and 7 are tables showing a number of lamp properties as afunction of the number of hours of operation of low-pressure mercurydischarge lamps wherein the luminescent layers are formed by a mixtureof phosphors comprising either BAM coated according to the invention oruncoated BAM, as a function of the number of hours of operation of theselamps and as a function of the layer thickness of the coating on the BAMand FIG. 8 is a schematic view partly in section, of a low-pressuremercury discharge lamp employing a coated luminscent material of theinvention.

DETAILED DESCRIPTION OF THE DRAWING

The invention will now be described in greater detail with reference tothe figures of the drawing.

The low-pressure mercury discharge lamps to which the test resultsdepicted in FIGS. 1 to 7 relate are electrodeless low-pressure mercurydischarge lamps with a power rating of approximately 85 W. These lampswere provided with an ionizable plasma containing mercury and argon andwith a glass lamp vessel. A luminescent layer was provided on the lampvessel wall. The data shown in FIGS. 1 to 4 relate to lamps whoseluminescent layers contained only one luminescent material: BAM. Theselamps can be subdivided into four types. Lamps of the first type hadluminescent layers formed by phosphor particles coated with a Y₂ O₃layer of approximately 50 nm thickness heated in a reducing atmosphereat approximately 900° C. Lamps of this type are indicated with BAM+Y₂ O₃/900 in the Figures. Lamps of the second and third type had luminescentlayers formed by phosphor particles coated with an Al₂ O₃ layer ofapproximately 50 nm thickness. For the second type, the Al₂ O₃ layer washeated to 1000° C. after being provided, for the third type to 1050° C.Lamps of the second and third type are indicated in the Figures withBAM+Al₂ O₃ /1000 and BAM+Al₂ O₃ /1050, respectively. Lamps of the fourthtype were provided with luminescent layers comprising uncoated BAMparticles. These last lamps are indicated in the Figures with BAM ref.

The luminescent layers of the lamps to which the test results shown inthe tables shown in FIGS. 5-7 relate comprised three luminescentmaterials: yttrium oxide activated by trivalent europium (YOX),cerium-magnesium aluminate activated by trivalent terbium (CAT) and BAM.The lamps differed from the lamps in the preceding paragraph as regardsthe composition of the luminescent layers only. Both uncoated and coatedBAM particles were used for making the luminescent layers of theselamps. The coated BAM particles may further be subdivided into BAMparticles coated with 25 nm Al₂ O₃, BAM particles are coated with 50 nmAl₂ O₃, BAM particles coated with 25 nm Y₂ O₃, and BAM particles coatedwith 50 nm Y₂ O₃. The lamps whose luminescent layers contained these BAMparticles are indicated in the Tables with A125, A150, Y25, and Y50,respectively. The lamps whose luminescent layers contained uncoated BAMare indicated in the Tables with Ref. The BAM particles provided with anY₂ O₃ layer were heated in a reducing atmosphere to a temperature ofapproximately 900° C. The BAM particles provided with an Al₂ O₃ layerwere heated in a reducing atmosphere to a temperature of approximately1040° C.

The above layers were provided as follows.

50 nm Y₂ O₃ layer on BAM:

9.0 g Y(NO₃)3.5H₂ O and 75 g urea were dissolved in 350 ml deionizedwater. The solution was filtered through a 200 nm filter. The filteredsolution was added to 50 g BAM in 1 l water, after which water was addeduntil the total volume was approximately 1500 ml. The suspension washeated to 90° C. over half an hour and kept at this temperature untilthe pH value was approximately 7. After the suspension had cooled downto a temperature lower than 50° C. it was filtered. The filtrate waswashed and dried overnight at 125° C. The particles thus obtained wereheated in air at approximately 600° C. for 12 hours, after which a finalheating step was carried out in a reducing atmosphere.

50 nm Al₂ O₃ layer on BAM:

15.2 g Al(NO₃)3.9H₂ O, 7.4 g malonic acid, 16.7 g NH₄ Cl, and 101.3 gurea were dissolved in 350 ml deionized water. The solution was filteredthrough a 200 nm filter. The pH was set for approximately 4-5 by meansof diluted NH₄ OH. This solution was added to 50 g BAM in 1 l water,after which water was added until the total volume was approximately1500 ml. The suspension was heated to 90° C. over half an hour and thenkept at this temperature for approximately 1.5 hour until the pH valuewas approximately equal to 7. After the suspension had cooled down tobelow 50° C. it was filtered. The filtrate was washed and driedovernight at 125° C. The particles thus obtained were heated in air atapproximately 600° C. for 12 hours, after which a final heating step wascarried out in a reducing atmosphere.

Layers with a layer thickness of approximately 25 nm were obtained inthat the above preparation method was carried out with 100 g BAM insteadof 50 g.

In the graphs of FIGS. 1 to 4, the number of hours of operation of thelamps is plotted on the horizontal axis in units of 1000 hours.

FIG. 1 shows on the vertical axis the luminous efficacy η of the lampsin lm/W. It can be seen that the lamps provided with BAM coated with Y₂O₃ have the lowest decrement in luminous efficacy during the first 1000hours of operation. All lamps with coated BAM have a lower luminousefficacy than lamps with uncoated BAM at 0 hours. The decrement in theluminous efficacy, however, is much smaller for lamps with coated BAM,so that after approximately 1000 hours the luminous efficacy of lampswith coated BAM is higher than the luminous efficacy of lamps withuncoated BAM. With BAM coated with Y₂ O₃, the luminous efficacy ishigher than that of uncoated BAM after 100 hours already.

In FIG. 2, the Hg consumption of the lamps in mg is plotted on thevertical axis. Hg consumption is understood to mean the quantity of Hgabsorbed by the luminescent layer. It is apparent that the Hgconsumption in lamps with uncoated BAM is highest. The lowest Hgconsumption was found in lamps of the second type. In a separate seriesof experiments, the Hg consumption of lamps provided with BAM coatedwith an Y₂ O₃ layer of approximately 100 nm thickness was alsoinvestigated. It was found that Hg consumption after 1000 hours did notdiffer measurably from that of BAM provided with an Y₂ O₃ layer of 50 nmthickness.

In FIG. 3, the shift dX of the x-coordinate of the colour point of thelight radiated by the lamps is plotted on the vertical axis in units of0.001. In FIG. 4, the shift dY of the y-coordinate of the colour pointof the light radiated by the lamps is plotted on the vertical axis inunits of 0.001. It is evident that the shift in the x-coordinate of thecolour point after 500 hours, and also after 1000 hours of operation isapproximately equal for lamps provided with Y₂ O₃ -coated BAM and forlamps with uncoated BAM. The lamps provided with Al₂ O₃ -coated BAMshowed a substantially smaller shift. The shift dY of the y-coordinatewas lower in all lamps provided with coated BAM than it was in lampsprovided with uncoated BAM. The smallest shift in the y-coordinate wasfound in lamps of the third type.

The table shown in FIG. 5 shows the luminous efficacy η in lm/W forlamps whose luminescent layers contained YOX, CAT, and BAM, at 0, 25,100, 1000, and 4000 hours of operation. The luminous efficacy of thelamps whose luminescent layer contains coated BAM drops less stronglythan does the luminous efficacy of lamps with uncoated BAM in theluminescent layer. A 50 nm thick layer on the BAM leads to a less strongdecrement in the luminous efficacy than does a 25 nm thick layer, bothin the case of Y₂ O₃ and in the case of Al₂ O₃. It could be concludedfrom results of further experiments, not shown in the Tables, that layerthicknesses greater than 50 nm, in the case of Y₂ O₃, hardly achieved asmaller decrement in the luminous efficacy of the lamps.

The table shown in FIG. 6 shows the shift dX of the x-coordinate of thecolour point of each lamp for the same lamps as in Table 1, after 0, 25,100, 1000, 3000, and 4000 hours of operation, relative to the colourpoint of the relevant lamp after 100 hours of operation.

The table shown in FIG. 7 shows for the same lamps as in Table 1 theshift dY of the y-coordinate of the colour point of each lamp after 0,25, 100, 1000, 3000, and 4000 hours of operation relative to the colourpoint of the same lamp after 100 hours of operation. It is apparent thatthe shift in the colour point can be substantially reduced throughcoating of the BAM forming part of the luminescent layer with Al₂ O₃ orY₂ O₃.

FIG. 5 shows a low-pressure mercury discharge lamp 10 which employs acoated luminescent material of the invention. Here a tubular glass bulb12 has mounts 14 sealed into both ends thereof. Each mount 14 comprisesa vitreous portion sealed to the end of the bulb 12 having lead-inconductors 16 sealed therethrough and supporting refractory metal coilsprovided with conventional electron emitting material (not shown).Electrical connections for the lead-in conductors 16 are provided bycontact pins 20 which project from supporting base caps 22 positioned atboth ends of the lamp 10. On the inner wall of the bulb 12 a luminescentlayer 26 provided with a coating 28 of of the invention is provided.

What is claimed is:
 1. A coated luminescent material, the luminescentmaterial coated with a crystalline layer of a metal oxide of the formulaM₂ O₃ wherein M is a metal selected from the group consisting of Y andAl and the layer is provided on the luminescent material in thefollowing manner:first the luminescent material is added to a solutionof a salt of the metal M, then through homogenous precipitation acompound of the metal M is deposited on the luminescent material, theluminescent material, provided with said compound, is separated from thesolution, dried and heated in air and then heated in a reducingatmosphere.
 2. The coated luminescent material of claim 1, wherein theluminescent material, after being dried, is first heated in air at atemperature of between 200° C. and 600° C., and is then heated in areducing atmosphere at a temperature of between 600° C. and 1200° C. 3.The coated luminescent material of claim 2, wherein heating in thereducing atmosphere is carried out at a temperature of between 850° C.and 950° C.
 4. The coated luminescent material of claim 2, whereinheating in the reducing atmosphere is carried out at a temperature ofbetween 950° C. and 1200° C.
 5. A coated luminescent material as claimedin claim 1, wherein a portion of the Y atoms is replaced by rare earthmetal atoms.
 6. A coated luminescent material as claimed in claim 1 or5, wherein the thickness of the layer formed by the metal oxide M₂ O₃ ischosen to lie within the range of 25 nm-100 nm.
 7. A coated luminescentmaterial as claimed in claim 1 or 5, wherein the luminescent material isbarium-magnesium aluminate activated by bivalent europium.
 8. The coatedluminescent material of claim 1, wherein M is Y and a portion of the Yatoms is replaceable by rare earth metal atoms.
 9. A low-pressuremercury discharge lamp provided with a luminscent screen provided with acoated luminescent material, the coated luminescent material coated witha crystalline layer of a metal oxide of the formula M₂ O₃ wherein M is ametal selected from the group consisting of Y and Al and the layer isprovided on the luminescent material in the following manner:first theluminescent material is added to a solution of a salt of the metal M,then, through homogenous precipitation a compound of the metal M isdeposited on the luminescent material, the luminescent material,provided with said compound, is separated from the solution, dried andheated in air and then heated in a reducing atmosphere.
 10. Alow-pressure discharge lamp as claimed in claim 9, wherein M is Y and aportion of the Y atoms is replaceable by rare earth metal atoms.
 11. Alow-pressure mercury discharge lamp as claimed in claim 16, wherein thecoated luminescent material is barium-magnesium aluminate activated bybivalent europium.